Contrasting Storm-Time Radiation Belt Events With and Without Dropouts-The Importance of CME Shocks

This study compares three different geomagnetic storms (designated as storms 1, 2, and 3) observed by NASA's Van Allen Probes within the spacecraft's first 50 days in orbit. These storms were Coronal Mass Ejection (CME)-driven with minimum DST around -138, -100, and -106 nT, respectively. Storms 1 and 2 occurred following the arrival of fast-forward shocks, which compressed the magnetopause inward to about 6.5 and 7.5 R-E, respectively, as a result of the increase in solar wind dynamic pressure and density. The inward magnetopause motion helped contribute to a rapid depletion of MeV electrons across the entire outer belt. For the third storm, however, there was little or no dropout of MeV electrons in the heart of the outer belt during the storm main phase. This third storm was generated by a CME without an associated shock, and the magnetopause actually moved outward at the start of the storm, suppressing loss of electrons through the outer boundary. The study reveals that under certain solar wind driving conditions radiation belt electron dropouts may not occur, even during large geomagnetic storms (Dst_min < -100 nT).